Re: Minibot climb rate
 

Have you considered shifting the impact point behind the battery? If you're correct in your guess that inertial load from the battery is pulling you away from the pole, then you could just shift your pivot point at the top of the minibot. If your impact point is farther from the pole than the battery, then the inertia of the battery will push you more into the pole. It might not be the optimal solution, but it might be quicker to implement. Plus it'd preserve your current weight distribution.

If none of that made sense, sketch up a free-body diagram of the minibot as it makes contact with the pole. Assign an arbitrarily large upwards force at the center of mass of your minibot, balanced by a downwards force at your point of impact, and a horizontal force centered on your magnets. See what happens to the horizontal force as you move the impact point farther from the pole.

Re: Minibot climb rate
 

Provided it is reaching the same max velocity (no matter how long it accelerates), doesn't reaching max velocity sooner yield the fastest time up the pole? If it is linearly accelerating the whole way up, the average velocity is only 1/2 the max velocity. RC cars don't have gravity to consider I reckon.

HTH

Re: Minibot climb rate
 

I could be wrong, but I don't think in this case the time-to-climb is minimized by making the wheel diameters so large that the minibot is accelerating all the way to the top.

Since you have a working minibot with a measured time-to-climb, you could easily plug your weight and wheel diameter into this model and adjust the friction value until the model matches your measured time-to-climb. Then change the wheel diameter in the model and see what diameter results in minimum time-to-climb.

A side benefit of smaller wheel diameter is less motor current.


Absolutely.

Re: Minibot climb rate
 

Kevin,
Your suggestion is actually quite easy to follow and might be the final solution. Currently, the initial impact to the sensor plate is the tip of the actuator arm of our Stop/brake limit switch. That is followed by the impact of the frame of the minibot.
To shift the initial impact point behind the battery should be as simple as placing a small amount of surgical tubing across the back of the top of the battery and then moving the battery up so that it is flush with the top of the frame. This will do two things:

1) Pad the battery with a bumper that is made of legal material, and reduce the possibility of damage to the battery from being slammed into the sensor plate.
2) It will shift the pivot point outside the center of mass of the minibot and thus cause it to rotate into the pole instead of away from it, as you suggested.

There is a minor problem with this. To get the minibot to accelerate faster, you have to go with smaller "wheels". This is direct drive with no transmission or gearing to deal with.

As you stated, average velocity can be determined by max/2. But, with smaller wheels, max velocity goes down. (Here is where practice makes perfect. If we really wanted the absolute fastest minibot possible, we would have to make several different sizes of axles to find which gave the best time.)

Let me try a little back of the envelope discussion to help explain this.

Let's assume that max velocity is reached just as you reached the top of the pole. Max velocity reached was 8 ft./sec. So average would be 4 ft./sec and the 10 foot pole would have been climbed in 2.5 seconds.

Now, we reduce the "wheel size" so that we accelerate to max velocity quicker, but in doing so, also reduce our max velocity. Now max velocity is 6 ft./sec and is attained at 6 feet up the pole (ball park for discussion). The average velocity for that 6 feet ( (6 ft./sec)/2) = 3 ft/sec, and therefore, 2 seconds to travel. The remaining 4 feet of distance is traveled at a velocity of 6 ft./sec and takes .667 seconds.

So, 2.667 seconds to cover the same distance instead of 2.5 seconds. That is why you "should" accelerate the entire way.

These numbers are for discussion only and do not represent the actual values.


Either,

I will definitely run these values through your model and see what the result are.

As I said, the real test would be to test with multiple different "wheel" sizes and measure the actual result.
There are so many variables in this project, battery voltage, wheel tackiness, wheel friction against the pole etc., that sometimes trial and error just works out better.

Re: Minibot climb rate
 

And now I'll turn around and doubt your back of the envelope calculations. I keep seeing people in this thread declaring that the average velocity is half the maximum velocity if you reach the maximum at the top of the pole. But this is only true if you're accelerating at a constant rate.

This is not the case with a permanent magnet dc motor. With a pmdc, as speed increases, produced torque decreases. As torque decreases, your acceleration decreases. So you're going to spend longer accelerating from max/4 to max/2 than you did accelerating from 0 to max/4. This is going to make a significant difference in your calculations. I'd recommend playing with the model Ether linked to and seeing the effect of changing wheel diameters, because it's not as straightforward as you think.

Re: Minibot climb rate
 

I believe you are spot on, but, for the sake of discussion, I believe the example I used is close enough to understand the principal.

Now, for complete accuracy, or theoretically accurate results, then Either's model should definitely be considered.

I will do a couple more measurements for accuracy tonight and then run them through the model and see what happens. (Honestly, with Spring Break being next week and the school being closed, I am fairly certain the only changes we will be making are those that keep the bot on the pole :yikes: )

Based on the values I recall for weight and wheel diameter, the Either Model shows that a wheel diameter of .436" is optimal. Currently we have axles that are .375" in dia. and then there is a very pliable Silicon Surgical tubing over that. Uncompressed, the entire wheel is fairly close to .5" in dia. So, under the load of compression into the pole, we are fairly close to the given number. Again, I will verify this tonight.

Re: Minibot climb rate
 

Did you adjust the friction until the model agreed with your time-to-climb, and THEN adjust the wheel diameter to find the optimum? See excerpt below:

Re: Minibot climb rate
 

I set up the model by entering in values for weight that are fairly close, but probably not exact. I then entered my best guesstimate for our current wheel diameter. I set the time to 1.2 sec, which is our measured time. Once these values were set, I adjusted the friction value until the distance value matched our climb distance.

Your Excel file does not modify the TIME value, TIME is an input, but it does modify the DISTANCE value as each input parameter is modified.

Once I found a friction value that matched our climb distance, I left that value alone. After that , I re-iteratively modified the wheel diameter until I maximized the distance traveled in the same time of 1.2 sec.

Re: Minibot climb rate
 

Its all about the numbers I reckon. You are trying hit the sweet spot on the motor curve. The fastest minibot I've seen is from FIRST Team 148. I didn't measure it but it seemed to be in the 1s range. It sure looked like that thing hit max V quickly.

Good luck in St Louis!

Re: Minibot climb rate
 

That should be OK.

That should get you in the ballpark. If you are game, try this instead: once you've found the friction value as described above, modify the wheel diameter and observe the graph to read the climb-time* for the given (fixed) distance. Find the wheel diameter which minimizes the climb-time for that given distance.

If you don't want to mess with that, would you mind posting your present observed values for bot weight, wheel diameter, and climb-time so I can run them?


* you can change the value in cell A7 to change the scale of the graph if necessary

Re: Minibot climb rate
 

I don't mind at all. I believe the values I used were as follows:
2.35LB.
.436 dia (final), .5 original to find friction.
1.2 sec. climb time.

Re: Minibot climb rate
 

Oops, forgot to ask: what did you use for the climb distance (ie the distance climbed in 1.2 seconds). Also, just to be sure: you didn't use a "ramp" launcher did you? In other words, the minibot started out from a dead stop and climbed in 1.2 sec correct?

Re: Minibot climb rate
 

I should have known that was needed :o
95 inches.

Re: Minibot climb rate
 

Given your inputs of 2.35 lb minibot weight, 0.5 inch shaft diameter, 1.2 seconds climb time, and 95 inches climbed distance, here's what I came up with:

friction: 0.83 pound

optimum diameter: 0.43 inches

... which tends to support the conclusion that you are pretty close to the optimum diameter. The only puzzling thing is that the graph in the model looks like your bot velocity with your original .5 inch diameter has pretty much reached its peak value half way up the pole. Was your assessment that the bot continued substantial acceleration all the way to the top a subjective judgment or did you analyze data from a video?

(see attachments)

Re: Minibot climb rate
 

Very much subjective. I based the comment on visual and auditory observation.

The bottom line is, we are really close. Lightening will only help, and deployment will be the biggest key to consistent success.